Wireless communication systems are generally allocated a portion of the radio frequency (RF) spectrum for their operation. The allocated portion of the spectrum is divided into communication channels and channels are distinguished by frequency, time or code assignments, or by some combination of these assignments. Each of these communication channels will be referred to as conventional channels, and a conventional channel typically corresponds to a full-duplex channel unless otherwise noted. The establishment of a communication link in a communication system depends not only on the availability of a conventional channel but also on the quality of communication that will result from the use of a given available conventional channel.
In wireless communication systems, a conventional channel is used for communication between a base station (sometimes referred to as cell station) and a subscriber station (sometimes referred to as a personal station). A cell station provides coverage to a geographic area referred to as a cell and may be a point-of presence providing a connection between the subscriber station and a wide area network such as a Public Switched Telephone Network (PSTN). The underlying motivation for the use of cells in wireless systems is the ability to reuse a particular portion of the RF spectrum available in geographically different areas. The reuse of the frequency spectrum can introduce co-channel (intercell) interference between users in different cells that share a common conventional channel. If co-channel interference is not carefully controlled, it can severely degrade the quality of communications. System capacity is in general limited by interference because of the reduction in number of reusable channels of acceptable quality.
Each cell is organized about a cell station. The cell station includes multiplexing equipment for accepting incoming telephone landlines (i.e., voice or data lines) and multiplexing the incoming voice/data signals onto a radio frequency (RF) carrier that is broadcast by an antenna system over a region that the cell is designated to cover. Individual subscriber stations (e.g., handsets and the like) are each equipped to receive the broadcast modulated carrier and to demultiplex a specifically assigned channel of the carrier that carries the voice/data that is intended for a given receiver.
In a conventional wireless communication system, an assigned RF bandwidth of frequencies is simultaneously shared by multiple subscribers. Three techniques for sharing bandwidth are frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA). In FDMA systems, the available bandwidth is sub-divided into a number of sub-bands. Each sub-band accommodates a carrier that is modulated by a subscriber's data. In TDMA systems, time-sharing is used to multiplex multiple subscribers. Each subscriber is allocated a periodic time-slot for transmission of data. In CDMA systems, multiple subscribers are accommodated on a single carrier (or sub-carrier) and each subscriber is assigned a code waveform that is used to modulate the carrier for each bit of data being transmitted. Each subscriber has an assigned coded waveform taken from a set of orthogonal waveforms, thus allowing the system to separate (demodulate) the individual subscriber transmissions.
Cellular communication systems may also use spatial division multiple access (SDMA) techniques for providing increased subscriber system capacity in systems that use FDMA, TDMA, and/or CDMA methods without any increase in the allocated RF bandwidth. SDMA techniques are discussed in greater detail in U.S. Pat. No. 5,515,378, to Roy III, et. al., entitled “Spatial Division Multiple Access Wireless Communication Systems.” SDMA exploits the spatial distribution of subscribers in order to increase the usable system capacity. Because subscribers tend to be distributed over a cell area, each subscriber-cell station pair will tend to have a unique spatial signature characterizing how the cell station antenna array receives signals from the subscriber station, and a second spatial signature characterizing how the cell station antenna array transmits signals to the subscriber station. Subscribers sharing the same conventional channel on a unique basestation are said to be using different spatial channels. The necessary data (referred to as the spatial signature of a subscriber) for implementing SDMA is obtained empirically from the transmissions received by the cell station from each active subscriber. Where spatial signatures are used, the effective radiation patterns of the antenna array can allow more than one subscriber to use a given packet time-slot, code or frequency. For example, if the effective radiation pattern of a first subscriber results in a relatively low energy “null” in the vicinity of a second subscriber sharing a packet time allocation, and the second subscriber's spatial signature results in a null in the vicinity of the first subscriber, the simultaneous RF packet transmissions will not cause interference upon reception at the two subscriber stations. Also, transmissions from the two subscribers to the cell station will be separable at the cell station.
A conventional wireless communication system includes a finite number of channels on which signals are transmitted. The number of channels depends on many system factors. By sharing a channel among subscribers, as discussed above with respect to SDMA techniques, more subscribers can be accommodated.
A particular example of an existing protocol for establishing a connection in a cellular communication system between a subscriber station and the cell station is described in “Personal Handy Phone System” which is part of the Association of Radio Industries and Businesses (ARIB) Preliminary Standard, Version 2, RCR STD-28, approved by the Standard Assembly Meeting of December, 1995.
In accordance with the PHPS standard, a control sequence is used to set-up and establish an incoming call to a subscriber station (i.e., a personal station or PS). The sequence includes:(1) the CS paging on a paging channel (PCH) of the selected PS to which an incoming connection is desired; (2) the selected PS responding on the signaling control channel (SCCH) by sending a link channel establishment request; (3) the CS responding to the PS request by selecting a traffic channel (TCH) and sending the selected TCH as a link channel (LCH) assignment to the PS on the SCCH; (4) the selected PS switching to the assigned LCH and transmitting a sequence of synchronization (SYNC) burst signals followed by a sequence of idle traffic bursts; and (5) upon successful detection of a synchronization signal, the CS responds by transmitting a sequence of SYNC bursts on the LCH followed by a sequence of idle traffic bursts and then proceeding to establish a connection with the incoming call to the CS, invoking any additional optional signaling that may be required (e.g. encryption and user authentication).
The control sequence for establishing an uplink connection initiated by a PS desiring to connect to the CS includes:(1) the PS sending a link channel establishment request on the signaling control channel (SCCH); (2) the CS responding to the PS request by selecting a traffic channel (TCH) and sending the selected TCH as a link channel (LCH) assignment to the PS on the SCCH; (3) the PS switching to the assigned LCH and transmitting a sequence of synchronization (SYNC) burst signals followed by a sequence of idle traffic bursts; and (4) upon successful detection of the synchronization signal, the CS responds by transmitting a sequence of SYNC bursts on the LCH followed by a sequence of idle traffic bursts and then proceeding to establish a connection with the incoming call to the CS, and invoking any additional optional protocols that may be required (e.g. encryption and user authentication).
In systems that use SDMA techniques, the control sequences described above can be modified depending on the number of subscribers being serviced and the number of channels available. For example, if a connection is sought to add a subscriber when there are no available channels (i.e., all available channels are assigned to subscribers), the sequence may be augmented to include a channel sharing selection process. One example of a channel sharing selection process is described in the commonly owned U.S. Pat. No. 5,886,988, entitled “CHANNEL ASSIGNMENT AND CALL ADMISSION CONTROL FOR SPATIAL DIVISION MULTIPLE ACCESS COMMUNICATION SYSTEMS,” the contents of which are expressly incorporated herein by reference. When a new subscriber is added, a sharing decision is made as to which current subscriber is the best match for pairing with the new subscriber. The sequence includes an assignment of the new subscriber to the channel occupied by the selected current subscriber, forming a best match.
While spatial channels can be used to increase the traffic managed per cell station, the use of spatial channels also increases the risk of call quality degradation and even call drop. Conventional systems assign new users or existing users locations for transmission consisting of a time slot and a frequency. Every transmission location has a risk of interference associated with it. Conventional systems manage these risks by monitoring various combinations of time slots and frequency to evaluate which location poses the least risk of interference to both the basestation and the phone. If the basestation incorrectly evaluates risk it might assign a call to a location that has a high level of interference causing performance problems or call drop. Basestations currently move calls around to different locations but only when the call quality starts to suffer.
When SDMA techniques are used, making a best pairing decision becomes paramount to performance. If not careful, a new subscriber may be assigned to a cell station and a channel on which poor quality is experienced due to excessive interference from the signal transmitted to a co-user. Moreover, the addition of a new subscriber has the potential consequence of adversely affecting the quality of communications on existing connections. Existing subscribers can suffer from increased channel interference from the addition of a new subscriber, or other unrelated causes, that can require moving subscribers from currently assigned channels to new channels in order to restore acceptable quality communications.
As described above, the spatial signature data collected for implementing SDMA and making the pairing decisions is obtained empirically from the transmissions received by the cell station from each active subscriber, including the new subscriber. However, the transmissions from the new subscriber necessarily are limited in nature (i.e., the new subscriber has been connected to the CS for only a small amount of time) and, as such, selections based on this limited amount of data may be less than optimal. The transmission characteristics of existing subscribers tend to be easier to quantify due to the length of time the connections have been set up. Further, some calls may be so short lived that the pairing of a new subscriber with the short call subscriber may be not desirable.